1. Field of the Invention
The present invention relates to a pilot operated electromagnetic valve.
2. Description of the Related Art
An automotive air conditioner is generally configured such that it includes a compressor, a condenser, an evaporator, and so forth arranged in a refrigerant circulation passage. Various types of control valves are provided for the purpose of, for example, switching the refrigerant circulation passages according to the operation state in such a refrigeration cycle and regulating the flow rate of refrigerant. A pilot operated electromagnetic valve capable of controlling the opening and closing of a large valve section using a relatively small electric power may be used as such the control valve (see Reference (1) in the following Related Art List, for instance).
Such an electromagnetic valve drives a small pilot valve element by a solenoid so as to open and close a pilot valve and then drives a large main valve element by a pressure difference regulated thereby so as to open and close a main valve. A piston is formed integrally with the main valve element, and a back pressure chamber is formed by this piston as a separated space inside a body. A leak passage through which to introduce the refrigerant into the back pressure chamber, and a pilot passage through which the refrigerant is led out from the back pressure chamber, are formed in the main valve element. And the opening and closing of the pilot valve opens and blocks the pilot passage, respectively. With this structure, the opening and closing of the main valve is controlled by varying the pressure of the back pressure chamber. The pressure of the back pressure chamber is regulated through a balance between the flow rate of refrigerant introduced into the back pressure chamber and the flow rate of refrigerant led out from the back pressure chamber.
2. Related Art List    (1) Japanese Unexamined Patent Application Publication (Kokai) No. 2001-124440.
As one of such electromagnetic valves, there is available a valve where a flexible sealing member, such as rubber, is placed at a tip of the pilot valve element to ensure the sealing property of the pilot valve and where the pilot valve element touches and leaves a pilot valve seat provided in the main valve element. By employing such a configuration as described above, the sealing member is appropriately deformed and then adheres tightly to the pilot valve seat when the sealing member seats on the pilot valve seat. Thereby, excellent sealing property can be achieved. Nevertheless, the pilot valve element is formed integrally with a movable iron core of the solenoid and therefore such a deformation may adversely affects the stroke of the movable iron core. In other words, the stroke of the movable iron core varies depending on a deformation amount of the sealing member and therefore the magnetic gap between the movable iron core and a fixed iron core varies. This may adversely affect the control performances of the solenoid. In some cases, the movable iron core and the fixed iron core hit each other and this may possibly cause noise. Also, depending on the shape of the pilot valve seat, the pressure-receiving diameter of the sealing member changes as a result of a change in the stroke of the pilot valve element and then the balance between a pressure difference exerted, between an upstream side and a downstream side of the pilot valve element, and the suction force of the solenoid is lost. This may also adversely affect the control performances.